1. Field of the Invention
This invention relates to an oxygen sensor for cleaning exhaust emissions from internal combustion engines, and more particularly to a lean sensor capable of sensing low concentrations of oxygen.
2. Description of the Prior Art
It is well known to use an oxygen sensor for cleaning exhaust emissions from internal combustion engines, particularly automobile engines.
One known oxygen sensor makes an efficient use of a phenonmenon wherein a solid electrolyte permits oxygen ion to permeate therethrough when a voltage is applied across the solid electrolyte. Namely, the oxygen concentration in the exhaust gas is detected from a threshold electric current by limiting the rate of flow of oxygen gas into the solid electrolyte. This type of oxygen sensor can easily be produced by forming electrode layers on both sides of a tabular solid electrolyte for applying a voltage across the latter, and providing a heat-resistant inorganic coating layer on at least one of the electrode layers. In addition, it is not necessary to use any reference gas as has been necessary in the conventional oxygen sensors. For these reasons, the oxygen sensing element of this type can be made extremely small in size. Furthermore, this type of oxygen sensor can be used as a lean sensor capable of sensing an extremely low concentration of oxygen.
In the production of the oxygen sensor of the type described above, however, special provisions must be made for attaching of the lead wires, supporting of the sensor element by a support and for preventing short-circuiting by carbon, because the sensor element has an extremely small diameter of 3 to 20 mm and a small thickness of 0.3 to 2 mm. In the conventional oxygen sensor of this kind, the output lead line is directly connected to the surface electrode on the element, so that the mechanical strength of this connection is extremely low, resulting in decreased reliability of operation against vibration and impact. In addition, the known oxygen sensor of this kind has suffered a problem of lack of stability of the support of the element and short-circuiting due to carbon.
A lean sensor as shown in FIGS. 5 and 6 has been proposed to obviate the above-described problems of the prior art. This lean sensor has a solid electrolyte element 1 made of zirconia stabilized by yttrium oxide or the like. The element 1 has a tubular form and a thickness ranging between 0.3 and 2 mm, and is provided at both its sides with electrodes 2 and 3 made of, for example, platinum. The element 1 is disposed between support plates 4 and 5 which are made of an insulating material having a high mechanical strength and high resistance to heat, e.g. alumina. The plate 4 and the plate 5 are bonded to each other by a heat-resistant bond 14 such as an inorganic bond. The plates 4 and 5 are provided with windows 4' and 5' of an area slightly smaller than the area of the element 1. After the element 1 is clamped between the plates 4 and 5, the windows and the portions of the plates 4, 5 around the windows are coated by coating layers 15 such as of alumina spinel which covers the element 1. Thin metallic films 8 and 9 such as of platinum are formed on the surfaces of the plates 4 and 5 adjacent to the element. The thin metallic film 8 is connected to the element electrode 2 while the thin metallic film 9 is connected to the element electrode 3 thereby to serve as lead wires. Thin metallic films 10 and 11 are formed on either one or both of the plates 4 and 5, and are coated by protective layers 12 and 13 formed thereon. Thin metallic films 10 and 11, made of platinum or the like, serve as heaters for heating the element when supplied with electric power or voltage.
In this lean sensor, however, the element cannot be located in a stable manner because the gap between the plates 4 and 5 is filled with a bond 14 by which the element 1 is fixed. In addition, since the space between two support plates is filled solely with the bond, the bonding strength between two support plates is so small that the mechanical strength of the sensor cannot be improved sufficiently. For incorporating the heaters 10 and 11 in the sensor, it is necessary to form protecting layers 12 and 13 on these heaters 10 and 11. This inconveniently makes it difficult to reduce the size of the sensor.